Induced radiative effects such as Raman scattering and fluorescence have become extremely valuable tools associated with the non-destructive determination of molecular constituents. To characterize a composition in a remote or hostile environment, optical fibers may advantageously be used to deliver excitation energy to a sample under investigation and to carry scattered radiation back to instrumentation for spectral analysis. An excitation source path may take the form of a laser providing a stimulus at an appropriate wavelength coupled to an input fiber, and a collection path may be made up of a second fiber carrying return radiative information to a spectral analysis tool such as a spectrograph.
In some instances, laser energy is now transported over relatively long distances via fiber optic cable. Such laser installations are increasingly used in industrial applications for materials processing, process monitoring, and process control. As an example, industrial Raman spectroscopy for chemical process monitoring and control may use laser energy from a laser source installed in a central control room instrument. The instrument couples the laser energy into an optical fiber cable that is routed to a remote probe head. The remote probe head is typically installed into a pipeline that may be hundreds of meters away from the laser source.
Such remote spectral analysis presents technical challenges, however, including the strong scattering signature of the material used for the optical fiber, this interference potentially being generated by both the laser excitation in the illumination fiber and any strong Rayleigh (unshifted) scattering allowed to enter the collection fiber. These spurious fiber signatures can compete with, or even overshadow, the desired signature of the sample under test, particularly when long lengths of fiber are used.
Raman spectroscopy is nevertheless gaining increasing acceptance in on-line process monitoring, due in large part to developments in instrumentation and associated component technologies. For a number of process applications, Raman analyzers have demonstrated significant advantages over alternative techniques such as gas chromatography, IR spectroscopy, and NIR spectroscopy. As a non-destructive, real-time technique, Raman spectroscopy is compatible with a wide variety of samples including opaque solids, aqueous solutions, emulsions, and gases, without the need for sample preparation.
Sampling in a process environment is most conveniently accomplished using a probehead assembly, as shown in the prior-art system of U.S. Pat. No. 5,377,004, incorporated herein by reference. Delivery of the excitation laser beam to the sample under test is accomplished via an excitation fiber-optic cable, and scattered light from the sample is collected by the probehead and routed back to the analyzer via a separate collection fiber-optic cable.
Various optical elements are inserted into the beam delivery path to remove the fiber signature, and the Rayleigh line is typically removed by a notch filter in the collection path. A beam combiner may serve to combine the laser beam delivery path onto a common optical axis with the collection path, so that a common sampling optic may be used for both paths.
Modern Raman instruments may also be configured to monitor multiple sample points in a process. In a typical industrial installation, multiple remote probeheads are coupled to a central instrument via separate fiber optic cables. The central instrument typically houses a laser source, spectrograph, CCD detector and control electronics. A sequencer or splitter 320 is used to multiplex the output of the laser source.
Widespread acceptance of Raman spectroscopy in chemical process monitoring requires accurate and timely instrument calibration. Key parameters to be calibrated in a Raman analyzer include the spectrograph wavelength axis and the laser wavelength itself. Calibration of the spectrograph wavelength axis determines the wavelength versus pixel mapping function of the spectrograph/camera assembly. There are a number of known wavelength calibration sources applicable to Raman spectrometers. Atomic emission lines from readily available neon or argon lamps form convenient wavelength calibration sources. Neon is preferable in that it provides emission lines in close proximity to the common 785 nm and 532 nm laser lines used in process Raman. A neon emission can also provide reference lines near both edges of the CCD for gratings used in certain types of commercially available Raman analyzer equipment.
However, since Raman detection is a frequency-shift phenomenon, wavelength calibration of the spectrograph alone is not sufficient for analyzing Raman shifts with the greatest possible accuracy. Calibrating the wavelength or frequency of the excitation laser source is equally critical. While gas lasers such as helium-neon or argon-ion lasers emit precisely known atomic emission lines, the emission wavelengths of the solid-state lasers more common in process Raman are less stable, and therefore require frequent wavelength calibration.
U.S. Pat. No. 6,351,306, incorporated herein by reference, resides in methods and apparatus for calibrating remote optical probe configurations of the type wherein a spectrum emitted by a sample is delivered to a spectrograph for analysis. The teachings are applicable to various spectroscopic techniques, including fluorescence and Raman detection. Depending upon the embodiment, the system and processes may be used to calibrate the spectrograph wavelength axis, the system spectral response or intensity axis, and the wavelength of the laser used for excitation.
The invention of the '306 patent is applicable to a variety of configurations, including process monitoring environments in which a plurality of probeheads and collection optical fibers are used, each one being associated with a different sample or portion thereof. Nevertheless, particularly in view of recent improvements in optical components and more demanding applications, there remains a constant need for enhancements to systems of this kind, including an ongoing need for multi-channel spectrometers with accurate and automated self-calibrating, diagnostic and safety features.